One of the popular arguments against mulching landscape and garden soils is that mulch delays soil warming and thus retards plant growth. Given that a well-chosen mulch will moderate temperature extremes – both hot and cold – is this an argument supported with evidence? In today’s post, I’m reporting the data I collected in visiting various parts of my home landscape and gardens and measuring soil temperatures.
For measurements, I used a soil thermometer placed at the same depth in every soil tested. This required movement of mulch if mulch was present, so that thermometers were inserted completely into the soil. These thermometers read the entire length of the probe, so readings represent the average temperature in the top 5” of soil. I took close-up photos of each of the areas tested. I took 5 measurements for each location.
Our evening temperatures have been near or below freezing, but the past several days have been sunny and the air temperatures are well into the 50F range. On March 17, it was 68F at 2 pm in the sun, though it was 27F that morning. The morning after (March 18), it was 35F.
There are several interesting trends to see on the box-and-whisker graph:
- Mulched raised beds have the most consistent temperatures, with no differences seen at any time or in any location measured.
- Unmulched soil mounds have extreme changes, mirroring air temperatures.
- Bare soil in beds under sunny conditions have extreme changes mirroring air temperatures, but not as great as that in raised beds. They are warmest during the day and coldest during the night.
- Bare soil in beds under shaded conditions are the coldest soils during the day and even colder at night.
- Soil under living mulch (turf) and beds with varying depths of wood chip are cooler during the day than bare soil in sunny conditions, but warmer at night.
- Bare soil in beds that were newly mulched are much warmer than bare soils not near mulched areas.
- The soil temperature under turf or in beds at least partially mulched did not change at night (data not shown on graph).
Extreme temperature swings can result in the death of germinating seeds, seedlings, expanding buds, and other tissues that aren’t cold hardy. This is especially true of tissues near the soil surface, where temperature are colder than they are at increased depths. Unprotected soil mounds show huge daily vacillations; comparative raised structures under mulch are cooler during the day but warmer at night. And bare soil in the shade is colder than any mulched soils. Consistency is important for young tissues, as they have few protections against environmental extremes.
What my little experiment demonstrates is what mulch research has consistently shown: appropriate mulch materials will moderate soil temperature extremes due to air temperature fluctuations. Just because a bare soil is 55F in the daytime doesn’t mean it won’t be 35F at night.